Temperatures in work? UK

So I’m 8wks 3days pregnant and my work leaves the door open due to the disabled access button not working and we are right on the street. Earlier this year the ceiling fell in and we had to close for 4 weeks. They didn’t replace the heating system. We have no heating whatsoever. It’s between 0-7 degrees Celsius where I live. The store manager (I work in a post office in the back of the store) has emailed the area manager but they take forever to fix things due to money. I don’t know the exact temperature in store I’m thinking of buying a thermometer to check but even customers are complaining it is so cold. Does anyone know employment law regarding this? I don’t think it is suitable for anyone not just me because I’m pregnant but maybe they would crack down on it even more? (I’m not planning on telling my work until 12 weeks)